Invalid Argument Error Python
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minute: Sign up Which exception should I raise on bad/illegal argument combinations in Python? up vote 234 down vote favorite 25 I was wondering about the best practices for indicating invalid argument combinations in Python. I've come across a few situations where you have a function like so: def import_to_orm(name, save=False, recurse=False): """ :param name: Name of some external entity to
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import. :param save: Save the ORM object before returning. :param recurse: Attempt to import associated objects as well. Because you need the original object to have a key to relate to, save must be `True` for recurse to be `True`. :raise BadValueError: If `recurse and not save`. :return: The ORM object. """ pass The only annoyance with this is that every package has its own, usually slightly differing BadValueError. I know that in Java there exists java.lang.IllegalArgumentException -- is it well understood that everybody will be creating their own BadValueErrors in Python or is there another, preferred method? python exception arguments share|improve this question edited Nov 2 '11 at 19:26 sth 127k33204309 asked Nov 1 '08 at 23:02 cdleary 27.4k40130178 add a comment| 4 Answers 4 active oldest votes up vote 242 down vote accepted I would just raise ValueError, unless you need a more specific exception.. def import_to_orm(name, save=False, recurse=False): if recurse and not save: raise ValueError("save must be True if recurse is True") There's really no point in doing class BadValueError(ValueError):pass - your custom class is identical in use to ValueErro
This module never needs to be imported explicitly: the exceptions are provided in the built-in namespace as well as the exceptions module. For class exceptions, in a try python exception message statement with an except clause that mentions a particular class, that clause
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also handles any exception classes derived from that class (but not exception classes from which it is derived). Two python errno exception classes that are not related via subclassing are never equivalent, even if they have the same name. The built-in exceptions listed below can be generated by the interpreter or built-in functions. http://stackoverflow.com/questions/256222/which-exception-should-i-raise-on-bad-illegal-argument-combinations-in-python Except where mentioned, they have an "associated value" indicating the detailed cause of the error. This may be a string or a tuple containing several items of information (e.g., an error code and a string explaining the code). The associated value is the second argument to the raise statement. If the exception class is derived from the standard root class BaseException, the https://docs.python.org/2/library/exceptions.html associated value is present as the exception instance's args attribute. User code can raise built-in exceptions. This can be used to test an exception handler or to report an error condition "just like" the situation in which the interpreter raises the same exception; but beware that there is nothing to prevent user code from raising an inappropriate error. The built-in exception classes can be subclassed to define new exceptions; programmers are encouraged to derive new exceptions from the Exception class or one of its subclasses, and not from BaseException. More information on defining exceptions is available in the Python Tutorial under User-defined Exceptions. The following exceptions are only used as base classes for other exceptions. exception BaseException¶ The base class for all built-in exceptions. It is not meant to be directly inherited by user-defined classes (for that, use Exception). If str() or unicode() is called on an instance of this class, the representation of the argument(s) to the instance are returned, or the empty string when there were no arguments. New in version 2.5. args¶ The tuple of arguments given to the exception construc
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optparse Setting up a Parser Defining Arguments Parsing a Command Line Simple Examples Argument Actions Option Prefixes Sources of Arguments Automatically Generated Options Parser Organization Sharing Parser Rules Conflicting Options Argument Groups Mutually Exclusive Options Nesting Parsers Advanced Argument Processing Variable Argument Lists Argument Types File Arguments Custom Actions Navigation Table of Contents Previous: optparse - Command line option parser to replace getopt. Next: logging - Report status, error, and informational messages. This Page Show Source Examples The output from all the example programs from PyMOTW has been generated with Python 2.7.8, unless otherwise noted. Some of the features described here may not be available in earlier versions of Python. If you are looking for examples that work under Python 3, please refer to the PyMOTW-3 section of the site. Navigation index modules | next | previous | PyMOTW » Generic Operating System Services » argparse - Command line option and argument parsing.¶ Purpose:Command line option and argument parsing. Available In:2.7 and later The argparse module was added to Python 2.7 as a replacement for optparse. The implementation of argparse supports features that would not have been easy to add to optparse, and that would have required backwards-incompatible API changes, so a new module was brought into the library instead. optparse is still supported, but is not likely to receive new features. Comparing with optparse¶ The API for argparse is similar to the one provided by optparse, and in many cases argparse can be used as a straightforward replacement by updating the names of the classes and methods used. There are a few places where direct compatibility could not be preserved as new features were added, however. You will have to decide whether to upgrade existing programs on a case-by-case basis. If you have written extra code to work around limitations of optparse, you may want to upgrade to reduce the amount of code you need to maintain. New programs should probably use argparse, if it is available on all deployment platforms. Setting up a Parser¶ The first step when using argparse is to create a parser object and tell it what arguments to expect. The parser can then be used to process the command line arguments when your program runs. The parser class is Argument